The database file format has changed. CouchDB itself does yet not provide any
tools for migrating your data. In the meantime, you can use third-party scripts
to deal with the migration, such as the dump/load tools that come with the
development version (trunk) of couchdb-python.

Some refinements have been made to error handling. CouchDB will send 400 instead
of 500 on invalid query parameters. Most notably, document update conflicts now
respond with 409 Conflict instead of 412 Precondition Failed. The error code
for when attempting to create a database that already exists is now 412
instead of 409.

CouchDB 0.9 changes sections and configuration variable names in configuration
files. Old .ini files won’t work. Also note that CouchDB now ships with two .ini
files where 0.8 used couch.ini there are now default.ini and local.ini.
default.ini contains CouchDB’s standard configuration values. local.ini is
meant for local changes. local.ini is not overwritten on CouchDB updates, so
your edits are safe. In addition, the new runtime configuration system persists
changes to the configuration in local.ini.